This invention is directed to an audio noise suppressor system of the open end type, i.e., one in which the audio spectrum is divided into a number of bands, in each of which output is suppressed independently of the others if the signal within that band is insufficient to override or mask noise.
The term 37 noise" is generally used to describe that part of the output of a communication channel which is not present in the input to the channel and which is not dependent on the input. The definition is usually further limited to that portion of the output which is non-systematic (i.e. not periodic or coherent) in nature, e.g., thermal or white noise, in order to distinguish it from "hum" or "ripple" due to imperfect filtering in the power supply to the equipment, etc., and from signal information resulting from mechanical imperfections, e.g., turntable rumble.
The invention herein is directed to the suppression of the aforementioned non-systematic noise. The device thought to be the fore runner of such open ended systems to which in applicant's opinion the present invention is most closely related is generally credited to H. F. Olson and dates back to about 1947. The Olson device split the upper audio spectrum into bands an octave or less wide by means of adjacent bandpass filters.
In this system signals below some threshold level in each band were discarded altogether, and signals larger than some small factor above that threshold were reproduced at full amplitude. The threshold was established by means of some non-linear device, such as back-to-back diodes, which would not pass small signals; the system was adjusted so that the noise signals were just below the threshold.
The presence of the non-linear devices caused substantial harmonic distortion for signals not too far above the threshold. It was necessary, then, to have bandpass filters following as well as preceding the non-linear devices, in order to discriminate against the harmonic distortion products: the choice of bands an octave wide or less was dictated by that consideration. In addition, there were intermodulation products within the individual bands, which could not be eliminated.
A modification credited to Price employed a balanced clipping system which, by eliminating even-order harmonics from the low-level distortion products, enabled the widths of the bandpass filters to be greater than one octave. The main advantage was economic (fewer filters); the wider bands resulted in somewhat more serious in-band intermodulation.
In more recent years there has been a revival of the Olson approach in the Kenwood brand "de-noiser" device and the Phase Linear brand "autocorrelator". The Kenwood device inverts the principle of the Olson system and employs cascaded band-stop filters, which are bypassed when signals are strong, rather than parallel bandpass filters which are disabled when signals are weak. Each filter is controlled by a corresponding tuned amplifier; when the signal in the latter is above some threshold level, a symmetrical detector causes current to flow in diodes which shunt the corresponding band-stop filter, permitting the previously rejected portion of the total signal to be transmitted.
There is a manual threshold control, which is adjusted by ear until the noise component of the signal is just insufficient to open up the band-stop filters. Signals significantly above the threshold thus set will open the filters, and will therefore be reproduced at full amplitude.
The Kenwood device has two advantages over the original Olson system in principle, and a third in practice. First, the uniformity of the large-signal response is not dependent on critical matching of adjacent bandpass filters, so that the large-signal response is virtually flat. Second, the distortion introduced by the diodes shunting the filters (and functioning as variable resistors) is lower than that introduced by the clippers used in the Olson system; however, this distortion is still comparatively large, and is significant when the noise level (and thus the signal level at which the diodes are in the intermediately conducting condition) is comparatively high. Third, in practice the threshold control can be adjusted to suit the input signal (within the overall limitations of the device), rather than requiring the input level to be adjusted to suit the fixed diode threshold.
The disadvantages of the system relative to the Olson device are: it is much more difficult to make band-stop filters to produce high, uniform attenuation over a narrow band of frequencies than to achieve the same result by switching out a bandpass filter; and the system requires tuned amplifiers as well as filters. The first of these relates to performance, in that the "sloppiness" of the cascaded filters could cause attenuation to occur in adjacent portions of the spectrum in which there was nevertheless enough signal to override noise in the vicinity of the signal. This becomes intolerable if an attempt is made to achieve more than 10 db of noise attenuation. The second disadvantage relates to cost.
The Phase Linear brand system, despite its name, differs from the Kenwood device only in a few particulars. Its cascaded band-stop filters are comparatively broad twin-T resistance-capacity types, instead of inductance-capacity as in the Kenwood; and its tuned control amplifiers employ resistance-capacity feedback to obtain a bandpass characteristic, again instead of inductance-capacity tuning. The threshold control employs audio automatic gain control (described as "logarithmic amplification"). Functionally this system is quite similar to the Kenwood device except that it employs fewer band-stop filters.
In addition to the above described open-ended noise suppression system there have in the past been developed other open-ended systems termed a dynamic noise filter system. These systems recognized that the perception of sound at low intensities is to some degree an inverse function of bandwidth. Thus, low-level high-frequency signals are less well heard than medium-frequency signals at comparable levels.
Correspondingly, the effect of discarding low-level high-frequency signals is less perceptible than that of discarding high-level high-frequency signals. Thus the system was designed so that its bandwidth increased as signal increased from some low level to some intermediate level, and therefore the effect was to reduce perception of noise with comparatively little perceived loss of signal bandwidth.
Once the bandwidth reaches the maximum, of course, all the noise present in the original signal is reproduced; but this only occurs when the signal is so large that the noise is at least partially masked by the intense signal. This principle was employed in the "dynamic noise suppressor" system credited to H. H. Scott (1946). A more sophisticated embodiment of the same principle was that credited to R. S. Burwen ("dynamic noise filter"), during the present decade; in that version, the bandwidth was made a function of both signal level and frequency, so that relatively small high-frequency signals were as effective in opening up bandwidth as large medium-frequency signals.
These dynamic noise filters of Scott and Burwen in their original form suffered from "noise-pumping", i.e., an audible increase in noise level, during musical passages in which the low-frequency content was large enough to open up the bandwidth, but in which there was insufficient high-frequency content to mask the noise which was thus allowed to pass through the system. In Burwen's embodiment of the principle, the noise-pumping was much reduced by the frequency-dependent control system, but another problem, that of "noise trailing" or "swoosh", could not be eliminated. The names refer to a phenomenon in which, after abrupt cessation of a loud passage, noise is fully audible, and trails away to a relatively low level over an appreciable interval of time.
The phenomenon is the result of the necessity to make the decay time of the control system much longer than the attack time in order to avoid severe intermodulation of high frequencies by large low-frequency signals to which the system inevitably remains somewhat sensitive. As a result, although such devices perform reasonably well when the noise is to be reduced in fairly low to begin with, they produce random-seeming effects, which are generally less acceptable than the steady noise would have been, when the noise level is of the order of that obtained from constant-velocity disc records.
The open-ended systems mentioned above and the present invention have in common the advantage of not requiring that the material with which they are used be pre-processed in any way. They are thus of general usefulness, unlike the various compandor systems known in the art. However, they share a common disadvantage: that there is some loss of high-frequency signal when it is insufficient to mask noise fully; either that, or the threshold or suppression controls must be set so as to permit substantially more noise to be transmitted than the system's capabilities warrant. This is, of course, inherent in the fact that the systems tackle the noise problem head on instead of avoiding it by artificially raising the threshold of signal relative to noise, as the compandors do.
The Olson type related systems as well as the system of this invention have an important advantage relative to the Scott system. The signals controlling the high-frequency filters are of essentially the same narrow bandwidth as the filters themselves, and do not have to cover a large portion of the audio spectrum. Thus, the release time can be a great deal shorter, there being no danger of modulation of high frequencies by low. Therefore, noise trailing is not a factor in the performances of the aforementioned Olson related systems.
Of the latter more recent Olson related systems, i.e., the Kenwood and Phase Linear brand systems, they are limited in noise-reducing performance by the use of band-stop rather than band-pass filtering, and employ signal-control techniques of an essentially non-linear character. In addition, they are inherently relatively costly because of the requirement for tuning in the control system as well as in the signal section. These problems are avoided in the noise suppressor of the present invention which because of its construction has notably superior performance in the presence of heavy noise such as that of a constant-velocity disc record or early tape recording.
In summary, all the open-ended systems perform well in the presence of slight noise; however, the noise suppressor of the present invention produces significant improvements thereover when noise is severe.